The present invention relates to vapor permeation curable coatings and more particularly to the spray guns and allied equipment used therefor.
Vapor permeation curable coating traditionally are a class of coatings formulated from aromatic hydroxyl-functional polymers and polyisocyanate cross-linking agents wherein an applied film thereof is cured by exposure to a vaporous tertiary amine catalyst. Originally, in order to contain and handle the vaporous tertiary amine catalyst economically and safely, curing chambers were developed. Curing chambers typically are substantially empty boxes through which a conveyor bearing the coated substrate passes and in which the vaporous tertiary amine, normally borne by an inert gas carrier, contacts such coated substrate.
Such traditional vapor permeation curable coatings requirements have been altered by the vaporous amine catalyst spray method disclosed by Blegen in U.S. Pat. No. 4,517,222. Such vaporous catalyst spray method relies on the concurrent generation of an atomizate of a coating composition and a carrier gas bearing a catalytic amount of a vaporous tertiary amine catalyst. Such generated atomizate and vaporous catalytic amine-bearing carrier gas flow are admixed and directed onto a substrate to form a film thereof. Curing is rapid even at room temperature and the use of a curing chamber is not required.
The vaporous spray catalyst method necessarily employs a variety of spray guns and allied equipment therefor. Such spray guns can be hand operated or can be automated for robotic implementation thereof. Additionally, the vaporous spray catalyst method quite readily has been adapted to both conventional air atomizing and air-assisted spray guns as well as electrostatic spray guns. Despite the universal application of the vaporous spray catalyst method, certain problems have developed in adapting such technology to existing commercial coatings lines. One problem is that when the spray gun is not in constant use, the temperature of the gun will equilibrate to the ambient temperature. When the "cold" spray gun then is activated, the vaporous amine in the vaporous amine gas flow can condense within the amine spray path within the gun and especially at the spray cap within the gun. Such amine condensation then can be carried with the atomized paint flow and can cause spotting on the substrate being coated. In electrostatic spray guns, and especially when the vaporous amine line to the electrostatic spray gun is not properly insulated, a path of liquid amine droplets can form from the air cap of the spray gun down through the line towards the vaporous amine generator. If the electrical path to the electrostatic or corona-inducing needle is not adequately insulated from the vaporous amine gas flow, then an electrostatic charge can be conducted down the liquid amine path which results in the safety mechanism within the electrostatic spray gun system shutting off the gun, otherwise the electrostatic charge keeps draining away. On an automated assembly line, it will be appreciated that such an occurrence results in unacceptable down time of the assembly line.
While some conventional spray guns have adequate provision for sealing the electrical line path from the vaporous amine gas path, many conventional spray guns are not so-designed since conventional technology employs only air in such lines so that condensation of an electrical carrier is not a problem confronted by spray gun manufacturers. The present invention is addressed to the foregoing concerns which have arisen in implementation of the vaporous spray catalyst method as disclosed in the U.S. Pat. No. 4,517,222 cited above.